Image formation by electrophotography is generally performed by a process which includes forming an electrostatic image on a photoconductor (electrostatic image bearing member), developing the electrostatic image with a developer so as to form a visible image (toner image), transferring the visible image onto a recording medium such as paper, and fixing the transferred visible image onto the recording medium with application of heat, pressure, a solvent gas, etc. so as to obtain a fixed image (see PTL 1).
Regarding the developer, one-component developers for which magnetic toners or nonmagnetic toners are solely used, and two-component developers composed of toners and carriers are known. One-component developing methods are classified into magnetic one-component developing methods and nonmagnetic one-component developing methods, depending upon whether or not magnetic force is used to keep toner particles on a developing roller.
As for the toners, each toner is generally produced by a kneading pulverization method in which a thermoplastic resin is melt-kneaded along with a colorant, etc., and then finely pulverized and classified. Additionally, if necessary, inorganic fine particles or organic fine particles may be added to surfaces of toner particles, for the purpose of improving the fluidity and cleanability of the toner particles.
Recent years, the method of providing toner releasability without application of oil to a heat roll, and adding a release agent such as a wax to a toner for preventing the problem of fusion of the toner is generally employed. Here, the toner releasability with respect to the heat roll is greatly affected by the dispersed state of the wax in the toner.
When the wax is compatible with a binder resin of the toner, toner releasability cannot be sufficiently exhibited, and the wax can exist as domain particles, thereby exhibiting toner releasability. On this occasion, when the dispersion diameter of the domain particles is too large, the proportion of the wax localized near the surfaces of toner particles relatively increases; thus, the domain particles may aggregate, causing degradation of particle fluidity, the wax or a carrier may transfer to a photoconductor, etc. during long-term use, causing filming, and so it may be impossible to obtain favorable image quality. When the dispersion diameter of the domain particles is too small, the wax is finely dispersed to excess and thus adequate toner releasability may not be yielded.
In the kneading pulverization method, since it is difficult to control the dispersion diameter of the domain particles of the wax is liable to be present on fracture surfaces, the amount of the wax exposed at the toner surface is large and so the above problems such as degradation of particle fluidity and occurrence of filming may arise. Further, there exist the following problems: the toner obtained by the kneading pulverization method generally has a broad particle size distribution, varies in frictional chargeability and easily causes fogging and the like; also, it is difficult to obtain a small-particle-diameter toner, i.e., a volume average particle diameter of 2 μm to 8 μm for reasons related to production efficiency, and the demand for improvement in image quality can hardly be met.
Accordingly, toners obtained by granulation in an aqueous phase have received an attention. The toners have narrow particle size distributions, can be easily reduced in particle diameter, and obtain a high-quality, high-definition image, and are superior in offset resistance and low-temperature fixing ability due to high dispersion of a release agent such as a wax.
Also, the toners are superior in transfer ability due to their uniform chargeability, and favorable in terms of fluidity, which gives an advantage in terms of design of a developing device, for example, it is possible to design a hopper with more freedom and reduce the toque with which a developing roll is rotated.
As the toners obtainable by granulation in an aqueous phase, toners obtainable by a suspension polymerization method or an emulsion polymerization aggregation method (hereinafter also referred to as chemical toners) have been conventionally developed.
The suspension polymerization method is a method of obtaining toner particles by adding a monomer, a polymerization initiator, a colorant, a wax, etc. into an aqueous phase containing a dispersion stabilizer with stirring so as to form oil droplets, and then increasing the temperature to effect polymerization reaction. The suspension polymerization method can achieve reduction in the diameter of the toner particles. By the suspension polymerization method, it is difficult to make the wax appropriately present on the surfaces of the toner particles unless a dispersion stabilizer is used, because the wax tends to enter the oil droplets easily when the oil droplets are being formed; here, there is a problem in which if the dispersion stabilizer remains, it causes a decrease in chargeability. Only spherical shaped toner particles are obtained, there is a problem of cleaning.
As the emulsion polymerization aggregation method, there is, for example, a method proposed in which a polyester resin is used as a binder resin; fine particles obtained by subjecting the polyester resin to emulsion dispersion in an aqueous phase and then removing the solvent are aggregated with a dispersion formed by dispersing a colorant, a wax (release agent), etc. in an aqueous phase; and the aggregated matter is heated and fused so as to produce toner particles (see PTLs 2 and 3). In this method, the shape can be controlled by controlling a heat-fusing temperature and time. According to this method, since ultrafine particles are not generated, there is no loss of emulsification, and further, it is possible to produce a toner having a sharp particle size distribution without needing classification. However, when the fine particles obtained after the solvent removal are aggregated, mere aggregation of the fine particles leads to insufficient cohesion thereof, causing cracks or the like at interfaces after the cohesion. Therefore, a heating step for allowing the cohesion of the particles to proceed by heat is necessary. However, when the heating is carried out, blooming of a wax component finely dispersed in the toner particles may arise (the wax component may be deposited on the surfaces), the wax formed into spherical shape, and/or aggregation, etc. of finely dispersed particles of the wax may arise, thereby making it impossible to maintain the state in which the wax is finely dispersed in a sufficient manner. Especially in the case where a wax (release agent) having a low melting point is used, it easily melts in the heating step, and thus there is a problem in which favorable toner releasability cannot be secured and so there is a lack of suitability of the toner for oilless toner fixation with a heat roller.
Meanwhile, there has been proposed a method of adding, to a toner composition, wax fine particles which are covered or impregnated with a vinyl polymer by adding a polymerizable vinyl monomer and a water-soluble polymerization initiator to a wax emulsion to effect polymerization, when the toner composition is emulsified, so as to uniformly and firmly attach the wax fine particles to the toner surface (see PTL 4). However, this method requires polymerization of a wax emulsion and a polymerizable vinyl monomer; moreover, the glass transition temperature Tg of a resin contained in the wax fine particles is high; thus, there is a problem in which the toner is inferior in low-temperature fixing ability and releasability at low temperatures.
Meanwhile, there has been proposed a method in which a polymerizable monomer that contains a polar group-containing substance and a wax is subjected to suspension polymerization in water to produce a toner, and thus the toner contains a wax having a low melting point that is unable to be used for a toner produced by a pulverization method (see PTL 5). In this method, a pseudo-capsule structure is employed in which a nonpolar component such as a wax is not localized near the surfaces of toner particles, as opposed to a polar component, but covered with the polar component at the surfaces. However, the dispersion of the wax inside the toner particles is not analyzed and is therefore unknown.
Meanwhile, use of a toner has been proposed in which the amount of a wax contained therein is in the range of 0.1% by mass to 40% by mass, and the wax exposed at the toner surface accounts for 1% by mass to 10% by mass of the constituent compounds exposed on the toner surface (see PTL 6). The proportion of the wax exposed on the toner surface is measured by Electron Spectroscopy for Chemical Analysis (ESCA) and thus determined. However, analysis based upon ESCA can be performed only within approximately 0.1 μm in depth from the outermost surface of the toner, and thus it is difficult to know the dispersed state of the wax which lies further inside and suitably exhibits toner releasability in a fixing step.
There has been proposed a method for producing a toner, in which the toner is heated at the temperature range which is −10° C. of a glass transition temperature of the toner or higher, but lower than the glass transition temperature thereof +10° C. for surface treatment, in order to improve transfer ability. In this method, degradation of resistance to smear is not described, and unknown (see PTL 7).
On the other hand, when in a fixing device, a toner and the like adhere to and accumulate on a periphery of a fixing belt or fixing roller, fixing ability is degraded, and the toner, etc. further accumulates thereon, causing degradation of image quality. Thus, conventionally, various methods for suitably cleaning a periphery of a fixing roller have been proposed. Examples thereof include a roller method in which a cleaning member is brought into contact with a periphery of a heating roller; a felt method in which a cleaning member formed of felt is slidingly contact with a heating roller; and a web method, in which a periphery of a fixing roller is cleaned with a web in the process that the web wound around a feeding roller is rolled up using a winding roller. For example, in PTL 8, there is a description of a cleaning device for a fixing unit using a cleaning web, but no description of a toner which can be efficiently cleaned. Recently, especially, copiers for production printing have been significantly developed, and there has been an advance in fixing at low temperature, improvement of fixing speed, double face printing of images with high image area ratio, and dealing with various paper except those for office use. As a result, a fixing unit is hard to be cleaned. In these fields, when the toner obtained by the above-described method is used, image failure (granular smear) may occur, because an offset toner is not sufficiently cleaned with a cleaning member, adheres to a fixing belt, a fixing roller or a pressure roller, and then is transferred to a recording medium. The cleaning ability of the offset toner, and the low temperature fixing ability and transfer ability of the toner are not satisfied simultaneously.
Therefore, it has been desired to provide a method for constantly, stably, efficiently producing a toner, which can form high quality images and has excellent resistance to smear with occurring less filming, while keeping advantages of the chemical toner having a small particle diameter and particle size distribution and excellent fluidity, but such a method has not yet been provided at the moment.